(1) Field of the Invention
The present invention relates to a solution for the preparation of a spinning original dope which is used to manufacture polyethylene fibers, films and the like having a high strength and a high modulus of elasticity. In particular, it relates to a concentrated and uniform solution of an ultra-high-molecular-weight polyethylene.
(2) Description of the Prior Art
A so-called ultra-high-molecular-weight polyethylene having an intrinsic viscosity [.eta.] of 8 or more in a decalin at 135.degree. C. and having as noticeably high a molecular weight as about one million or more is utilized as an engineering plastic which can be characterized by excellent shock resistance and wear resistance as well as high self-lubricating properties. Therefore, the ultra-high-molecular-weight polyethylene is now used in the forms of hoppers, silos, various gears, lining materials, ski beacking materials and the like in many fields of food machines, civil engineering machines, chemical machines, agriculture, mining, sports/leisure and the like.
The ultra-high-molecular-weight polyethylene has a high molecular weight as compared with prevalent or conventional polyethylenes. In consequence, if a high orientation of the ultra-high-molecular-weight polyethylene is achieved, a stretched product having a higher strength and elasticity than ever can be procured. That is the reason why much attention is now paid to a variety of investigations on the technique of the above-mentioned high orientation. However, the ultra-high-molecular-weight polyethylene has a much higher melting viscosity than prevalent polyethylenes, and therefore it can scarcely be melt-extruded in a usual manner in a molding process and it also is impossible at present to achieve the high orientation by stretching.
On the other hand, Pole Smith, Pieta Yang Lemstra et al have suggested a method for preparing fibers having a high strength and a high modulus of elasticity by stretching, in a high magnification, a gel which had been obtained from a decalin dope of an ultra-high-molecular-weight polyethylene (U.S. Pat. Nos. 4,422,993 and 4,430,383). In this dope, a polymer of 1.5.times.10.sup.6 weight-average molecular weight is present at a concentration of 3 wt % and that of 4.times.10.sup.6 weight-average molecular weight is present at an extremely low concentration of 1 wt %. In the practice of the suggested technique, therefore, there is the problem that a great deal of solvent must be treated, economic problems and others. For this reason, a highly concentrated solution of the ultra-high-molecular-weight polyethylene is desired and demanded. As solvents which are useful for the preparation of the ultra-high-molecular-weight polyethylene solution, there have been suggested xylene, decalin, o-dichlorogenzene, trichlorobenzene and the like, as disclosed in U.S. Pat. Nos. 4,413,110, 4,422,993, 4,430,383 and 4,536,536 and European Pat. No. 139141, but they are not sufficient. Further, in order to prepare a more concentrated solution, a method of irradiating ultrasonic wave and a method of concentrating its dilute solution have been suggested, but in both of such methods, molecular chains tend to be cut. It is fair to say that the aforesaid methods are not preferable for the purpose that properties inherent in the ultra-high-molecular-weight polyethylene are elicited therefrom by the subsequent molding process.
As is definite from the above, it is never easy to prepare a uniform dope to be molded in a manner of dissolving the ultra-high-molecular-weight polyethylene in a solvent, which fact prevents the application of the above suggested process on an industrial scale.
In addition thereto, there is a method in Japanese Patent Provisional Publication No. 136820/1983 in which a double cylinder is employed to prepare fibers from an ultra-high-molecular-weight polyethylene solution. In this disclosed method, the ultra-high-molecular-weight polyethylene solution is placed in the double cylinder, and an inside cylinder is rotated at a high speed, while the solution is maintained at such a high temperature as not to crystallize by itself. In this case, when a fine yarn is brought into contact with the surface of the inside cylinder, the yarn functions as a seed for crystallization, so that a fibrous ultra-high-molecular-weight polyethylene crystal grows under the rotation of the cylinder. When the seed is drawn out from the system in compliance with the growth of the polyethylene crystal, a continuous fiber can be manufactured. Also in this method, it is crucial for its success to select a suitable solvent for dissolving the ultra-high-molecular-weight polyethylene therein.